AUTHOR=Gu Xueji , Cheng Fang , Chen Xiaolei , Du Guanxiang , Zhang Guiling TITLE=Dissolved Nitrous Oxide and Hydroxylamine in the South Yellow Sea and the East China Sea During Early Spring: Distribution, Production, and Emissions JOURNAL=Frontiers in Marine Science VOLUME=8 YEAR=2021 URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2021.725713 DOI=10.3389/fmars.2021.725713 ISSN=2296-7745 ABSTRACT=

Coastal marine systems are active regions for the production and emission of nitrous oxide (N₂O), a potent greenhouse gas. Due to the inherently high variability in different coastal biogeochemical cycles, the factors and mechanisms regulating coastal N₂O cycling remain poorly understood. Hydroxylamine (NH₂OH), a potential precursor of N₂O, has received less attention than other compounds in the coastal areas. Here, we present the spatial distribution of N₂O and the first reported NH₂OH distribution in the South Yellow Sea (SYS) and the East China Sea (ECS) between March and April 2017. The surface N₂O concentrations in the SYS and the ECS varied from 5.9 to 11.3 nmol L^–1 (average of 8.4 ± 1.4 nmol L^–1) and were characterized by offshore and north–south decreasing gradients. NH₂OH showed patchy characteristics and was highly variable, fluctuating between undetectable to 16.4 nmol L^–1. We found no apparent covariation between N₂O and NH₂OH, suggesting the NH₂OH pathway, i.e., nitrification (ammonium oxidation), was not the only process affecting N₂O production here. The high NH₂OH values co-occurred with the greatest chlorophyll-a and oxygen levels in the nearshore region, along with the relationships between NO₂^–, NO₃^–, and NH₂OH, indicating that a “fresh” nitrifying system, favoring the production and accumulation of NH₂OH, was established during the phytoplankton bloom. The high N₂O concentrations were not observed in the nearshore. Based on the correlations of the excess N₂O (ΔN₂O) and apparent oxygen utilization, as well as ΔN₂O vs. NO₃^–, we concluded that the N₂O on the continental shelf was mainly derived from nitrification and nitrifier denitrification. Sea-to-air fluxes of N₂O varied from −12.4 to 6.6 μmol m^–2 d^–1 (−3.8 ± 3.7 μmol m^–2 d^–1) using the Nightingale et al. (2000) formula and −13.3 to 6.9 μmol m^–2 d^–1 (−3.9 ± 3.9 μmol m^–2 d^–1) using the Wanninkhof (2014) formula, which corresponds to 75–112% in saturation, suggesting that the SYS and the ECS acted overall as a sink of atmospheric N₂O in early spring, with the strength weakening. Our results reveal the factors and potential mechanisms controlling the production and accumulation of NH₂OH and N₂O in the SYS and the ECS during early spring.